The Blue Screen

Chroma Keying is similar to using a Luminance Key. In Luminance Keying, the brightness level is set and all of the brighter (or dimmer, depending on the wishes of the camera operator) pixels are turned off. Then a second image can be slid in behind the actor that shows through where the pixels have been turned off.

Obviously, a Luminance Key is really only useful when creating greyscale images, such as in black and white imagery.

Chroma Keying works the same way, except it uses a color instead of a brightness level. The reason for this is that there are many more variations of color that can be used as opposed to limited black-white or brightness levels.

Traditionally, this color has been blue. Hence the term, "blue screen". A common color these days is also green, but theoretically any color is possible. Blue is most often used because the most common subjects filmed are people, and flesh tones stand out better against a blue backdrop than any other color. Additionally, when color spill or bleeding occurs, blue is easier to cover up than green or another color.

It important to note that when a color is properly "keyed out", ALL of that color will disappear. If your subject is wearing blue and you are Keying out blue, then the blue your subject is wearing will also Key out. This can leave "holes" in the actor where the background will show through.

In fact, this is exactly how Lt. Dan in the movie, "Forrest Gump" lost his legs during the film. The actor who plays Lt. Dan (Gary Senise) wore a pair of long blue socks, and when the blue was keyed movie magic occurred. His legs simply vanished.

Depending on your equipment, editing software or even personal preference, you can decide the color used for yourself.


Source: http://www.signvideo.com